There is an optical disk apparatus in which a light beam emitted from a light source, such as a semiconductor laser, is focused on a disk type recording medium rotating at a prescribed speed, and signals are recorded in, or reproduced from, the recording medium, as described in Japanese Published Patent Application No. Hei. 7-129968. The disk type recording medium has a spiral or concentric tracks. The width of the tracks is about 0.6 .mu.m, and the pitch of the tracks is about 1.5 .mu.m. In order to record signals on the tracks or reproduce signals recorded on the tracks, a focusing of the light beam irradiating the recording medium is controlled so that the light beam is focused on a prescribed position of the recording medium.
FIG. 19 is a block diagram illustrating an example of an optical disk apparatus including a focus control system.
The apparatus shown in FIG. 19 comprises a light source 1, such as a semiconductor laser, that emits a light beam 8 toward a disk 7 (recording medium), a coupling lens 2, a polarization beam splitter 3, a polarizing plate 4, a focusing lens 5, and a disk motor 6 for rotating the disk 7 at a prescribed speed. A light beam 8 emitted from the light source 1 is collimated by the coupling lens 2, and reflected by the polarization beam splitter 3 to the polarizing plate 4. The light beam then travels through the polarizing plate 4 and the focusing lens 5, and is focused on the disk 7 rotated by the disk motor 6.
This apparatus further comprises a condenser lens 9 and a split mirror 10 as elements receiving a light beam reflected at the disk 7. The reflected light beam travels through the focusing lens 5, polarizing plate 4, the polarization beam splitter 3, and the condenser lens 9, and is then split into two beams 11 and 15 by the split lens 10. The light beams 11 and 15 are applied to a focus control system and a tracking control system, respectively.
The focus control system comprises a two-element photodetector 12, a preamplifiers 13A and 13B, a differential amplifier 14, a phase compensator 18, a linear motor 19, a switch 33, a driving circuit 35, a focus control element (focus actuator) 36, a logic circuit 40, a comparator 41, and a chopping wave generator 42. The two-element photodetector 12 has two light responsive parts A and B. Output signals from the light responsive parts A and B are amplified by the preamplifiers 13A and 13B, respectively, and are inputted to the differential amplifier 14. A knife edge detection is realized by the condenser lens 9 and the split mirror 10, and a signal output from the differential amplifier 14 is a focus error signal (hereinafter referred to as FE signal).
The phase of the FE signal in the focus control system is compensated by the phase compensator 18, and the switch 33 opens or closes a loop of the focus control system. When the focus control system is closed by the switch 33, the FE signal output from the phase compensator 18 is sent to the driving circuit 35 through the switch 33. The driving circuit 35 amplifies the FE signal and sends the FE signal to the focus control element 36. In this structure, when the focus control system is in the closed state, the focus control element 30 is driven so that the light beam is always focused on a prescribed position of the disk 7. Further, an output signal from the chopping wave generator 42 is also input to the switch 33. The FE signal is also input to the logic circuit 40 through the comparator 41. The logic circuit 40 controls the opening and closing operation of the switch 33.
The linear motor 19 moves the focusing lens 5, the focus control element 36, the polarization beam splitter 3 and the like in the direction transverse to the tracks on the disk 7. The linear motor 19 is operated when the focal point of the light beam is moved to a prescribed track.
On the other hand, the light beam 15 from the split mirror 10 is input to the two-element photodectector 16 in the tracking servo system. The photodetector 16 has two light responsive parts C and D, and a difference between output signals from the respective light responsive parts C and D becomes a track error signal. The light beam on the disk 7 in controlled by this track error signal to correctly scan the tracks on the disk 7. Since the present invention does not relate to the tracking control, a detailed description of the tracking control is omitted here.
In the optical disk apparatus with the focus control system shown in FIG. 19, the focus control is performed as described below.
Initially, the disk 7 is rotated by the disk motor 6. When a prescribed rotating speed is reached, the switch 33 selects the chopping wave generator 42, and the focus control element 36 is operated in response to a signal output from the chopping wave generator 42, whereby the focusing lens 5 is moved up and down, i.e., in the direction perpendicular to the recording face of the disk 7. Thereby, the focal point of the light beam on the disk 7 moves up and down. At this time, an S-shaped FE signal (hereinafter referred to as S signal), which appears when the focal point of the light beam passes through the recording face, is detected by the comparator 41. By the detection of the S signal, the logic circuit 40 knows whether the focal point of the light beam is positioned in the vicinity of the recording face or not. When the focal point is positioned in the vicinity of the recording face, the logic circuit 40 controls the switch 33 to select the phase compensator 18. In this way, the focus servo loop is closed, and the focus control (focus lead-in) is performed so that the light beam is focused on a prescribed target position.
The focus lead-in will be described with reference to FIGS. 20(a), 20(b), 21, and 22. FIGS. 20(a) and 20(b) illustrate a waveform of a focusing lens driving signal and a waveform of an FE signal having S signals, respectively, at the focus lead-in. FIG. 21 illustrates a waveform for explaining the relationship between the focus lead-in level and S signals that appear in the FE signal at a protection film at the surface of the disk 7 and at the recording film when the focusing lens 5 comes close to and goes away from the disk 7. FIG. 22 is a flow chart showing a fundamental focus leadin procedure in the focus control system.
As shown in FIG. 22, when the reading and reproducing apparatus is turned on, the disk motor 6 is turned on and the disk 7 is rotated (step S21). When the disk 7 reaches a prescribed rotating speed, the light source 1 is turned on, and the semiconductor laser emits light (step S22). Subsequently, the linear motor 19 is driven to move the focusing lens 5 toward the inner circumference of the disk 7 (step S23). The above-mentioned initial operation is followed by the focus lead-in operation.
In the focus lead-in operation, initially, the focusing lens 5 is moved down away from the disk 7, in response to an output signal from the chopping wave generator 42 (step S24). Thereafter, the focusing lens 5 is moved up toward the disk 7 (step S25). While repeating the up and down movement of the focusing lens 5, it is detected that the S signal reaches a prescribed lead-in level (step S26). After the prescribed lead-in level is reached, the logic circuit 40 controls the switch 33 to select the phase compensator 18, and the up and down movement the focusing lens 5 is stopped (step S27). Then, the focus control system is turned on (step S28), the focus lead-in is ended, and the focus control is started.
The detection level (lead-in level) of the comparator 41 for the focus lead-in is normalized by the amplitudes of the S signals which are output due to the reflection at the recording film of the disk 7 and the reflection at the protection film. That is, as shown in FIG. 21, the focus lead-in level is set within a linear interval that is larger than the peak of the S signal at the protection film and between the peak of the S signal at the recording film and the zero level.
When the prior art focus lead-in process is applied to large capacity optical disks having two or more information faces as shown in FIGS. 6(a) and 6(b), for example, a digital video disk (hereinafter referred to as a DVD), S signals appear at every passing of the focal point of the light beam through each information face, so that S signals as many as the information faces appear when the focusing lens is moved up and down during the focus lead-in operation. For example, in a dual-layer DVD, as shown in FIG. 7, in addition to small S signals at the protection film, two periodic S signals appear on each respective information face. Therefore, in the prior art focus control system, when the S signal at the surface protection film is detected by mistake, the focus control turns on at that part and the focus lead-in ends in a failure. Likewise, when the focus control turns on at the two S signals on the information face, it cannot be detected on which one of the two information faces that the focus lead-in is performed. Therefore, it is very difficult to reproduce information by selecting one of the two information faces certainly and performing focus control and tracking control on the selected information face.
Further, in order to realize a compatibility between a DVD and a CD, an optical head shown in FIG. 1 includes a hologram element 106 that produces two focuses 107a and 107b. In this case, when the disk loaded in the apparatus has a single information face like a CD, an S signal appears at each focal point, so that it is difficult to decide at which one of the two focuses that the focus lead-in should be performed. When the disk loaded in the apparatus is a DVD having two information faces, at least six S signals appear in the FE signal at each UP or DOWN of the focusing lens 105 as shown in FIG. 7. Further, when the surface deflection of the disk is large, the S signals interfere with each other and become nonlinear. In this case, it is almost impossible to learn the lead-in level by measuring the amplitudes of the S signals and detect the information face on which the focus lead-in should be performed.
Furthermore, when the disk loaded in the apparatus is a disk having two or more information faces, the eccentricity, the focus offset value, the tracking off-set value, the focus gain value, the tracking gain value, and the focus error during the detection vary for each information face. Therefore, even though these correction values are appropriately set for one information face, when the light beam is moved to another information face for reproduction or recording of information, considerable focus error and track error occur on that information face, whereby the focus control and the tracking control become unstable. Further, in the detection of the of tracks, the focus error becomes significant because the light beam crosses the grooves, so that stable detection cannot be performed.
Furthermore, the prior art optical disk apparatus is not suited for a CD, a single-layer DVD, a dual-layer DVD, and a disk of write once read many type, such as a CD-R or a DVD-R. When such a disk is loaded in the apparatus, the apparatus indicates an error or the disk is compulsorily ejected from the apparatus.